1. Field of the Invention
This invention relates to a circuit for producing gating pulses in response to a pulse type input signal and more particularly to a circuit in which the gating pulses are produced only if the peaks of the input signal exceed a predetermined threshold level.
2. Description of the Prior Art
In a typical data readback circuit of a disk drive storage device, a transducer produces a pulse type readback signal in response to binary digital data recorded as transitions in a magnetic medium such as a magnetic disk of the disk storage device. The locations of the peaks in the readback signal are representative of the data. However, since the readback signal is also likely to contain peaks produced by spurious sources such as noise, the readback signal is usually applied to a detector circuit which processes the readback signal to produce a data representative signal devoid of noise. Noise is undesirable because it may cause shifting of the data peaks in the readback signal and thereby adversely affect the accuracy of detecting the data. The detector circuit typically comprises a peak detecting channel coupled in parallel with a gate generator channel. U.S. Pat. No. 4,081,756 issued Mar. 28, 1978, to Price et al discloses a dual channel detector circuit which functions in the manner of and which includes a gate generator circuit of the kind to which the present invention relates. As disclosed in the Price et al Patent, the gate generator (amplitude detector) circuit functions to produce gating pulses in response to each pulse of the read signal having a peak amplitude exceeding a predetermined threshold level. The threshold level is usually chosen to be the minimum amplitude level expected for a peak representative of data so as to preclude producing gating pulses in response to noise pulses having amplitudes less than the threshold level. The gating pulses are then applied to a gate circuit along with peak detected signals produced by the peak detector circuit. Since the gate generator produces gating pulses only in response to and in time coincidence with data pulses, noise pulses do not pass through the gate circuit. Detection errors can still occur, however, because the dual channel detector circuit includes circuits operating in parallel with one another and therefore circuit noise and other operating uncertainties in the respective circuits of each channel introduce timing errors which are likely to result in the detected peaks representative of data in the signal being randomly shifted out of time coincidence with its corresponding gating pulse. Accordingly, the pulse width (time duration) of each gating pulse must be sufficiently wide to compensate for timing errors in order to preclude detection errors. But the gating pulse must also be limited in pulse width to preclude detected noise peaks from being gated through the detector circuit and included in the data representative signal.
Gate generator circuits generally operate to produce gating pulses of either fixed or variable width time durations. In the case of fixed-width gate generator circuits, the time duration of a gating pulse is fixed to be sufficiently wide for detecting a prescribed nominal data pulse of the readback signal. Typically, an equalizer circuit and an automatic gain control (AGC) circuit, which are well known in the art, are used to process the readback signal to insure that pulses applied to the input of the dual channel detector circuit all have symmetrical waveshapes and amplitudes substantially equal the prescribed nominal level. However, in modern disk drive systems having a plurality of disks and associated transducers, the peak amplitudes of the readback signal may vary greatly because of variations in operating characteristics from transducer to transducer or variations in recording characteristics from disk to disk. Since AGC circuits cannot instantaneously respond to initial amplitude variations of the readback signal, pulses deviating from the nominal amplitude are likely to be produced for some transient period each time data is read back from a different transducer or disk than that used previously. Problems arise when a readback pulse is substantially greater or less than the nominal pulse. In a fixed-width gate generator, the gating pulse starts when the amplitude of the readback pulse applied thereto reaches the gate generator threshold and lasts for a predetermined (fixed) time interval so that the peak of a nominal readback pulse coincides with the center of its associated gating pulse. Therefore, when a larger than nominal readback pulse is applied to the input of the detection circuit, the detected peak of such pulse occurs after the midpoint (towards the trailing end) of the associated gating pulse. This result is undesirable because the aforementioned timing errors may further delay the peak so that the peak is shifted outside the fixed width of the associated gating pulse thereby resulting in loss of data. Alternatively, when a smaller than nominal pulse is applied to the input of the detector circuit, the detected peak of such small pulse occurs before the midpoint of its correspondingly produced gating pulse. Such is also likely to result in lost data because the timing errors may further advance the peak of the small pulse so that the peak is shifted outside the fixed width of its associated gating pulse.
However, if the gating pulse came on upon the crossing of the gate generator threshold and remained on until the amplitude of the readback pulse fell below the threshold, the problem described above would not occur. In the case of a variable-width gate generator circuit, the time duration of each gating pulse is typically dependent upon the time duration that the readback signal pulse exceeds a predetermined threshold level. Accordingly, a larger than nominal or a smaller than nominal readback signal pulse will result respectively in a larger or smaller than nominal gating pulse. In the absence of timing errors, the pulse peak would coincide with the midpoint of the associated gating pulse. Timing errors generally would not cause lost data problems in the situation involving larger than nominal pulses in the readback signal because the correspondingly large gating pulses produced and associated with such pulses are likely sufficiently wide to accommodate for the shifting caused by timing errors. However, in the case of smaller than nominal pulses in the readback signal timing errors are still likely to result in lost data because the durations of the associated gating pulses are too small to accommodate the relative shifting of the detected peaks.
Therefore, while the two types of gate generator circuits described above are satisfactory for readback signal pulses having a prescribed nominal waveshape, there is a need for improvement in the case where the readback signal pulses are substantially smaller than the nominal pulse.